Compensating arrangement for transmission lines



Aug. 21, 1951 s CRARY 2,565,297

COMPENSATING ARRANGEMENT FOR TRANSMISSION LINES Filed Oct. 5 1949 QRECEIVER GENERATOR A l sYsTEM Q? E Q 2 1t J swrrcHGEAR ELEMENT /a U /a m3 F [2 Fl RECEIVER GENEkATOR SYSTEM SWITCHG EAR ELEMENT His At torneythe series capacitors. therefore, series capacitors are required to bein Patented Aug. 21", 1951 COLIPENSATING ARRANGEMENT FOR TRANSMISSIONLINES Selden Crary, Schenectady, N. Y., assignor to General ElectricCompany, a corporation of New York Application October 5, 1949, SerialNo. 119,693

1 Claim.

This invention relates to electrical transmission lines and moreparticularly to the use of intermediate synchronous condensers forcompensation thereof.

The use of condensers in relatively long transmission lines to improvethe voltage regulation and stability thereof is well known. Onearrangement which has been employed is the use of capacitors in serieswith the transmission line to provide so-called series capacitorcompensation. Another arrangement is the use of intermediate synchronouscondenser stations at various points along a transmission line. Suchstations employ synchronous condensers connected across the transmissionline and are generally provided with automatically controlled fieldexcitation in combination with continuously acting static-type voltageregulators. Experience has shown that the use of such regulators withproperly co-ordinated excitation systems may substantially increase thepower limits of long distance transmission systems. It is withcompensating arrangements of the latter type, making use of intermediatesynchronous condensers, that the present invention is concerned.

A description of the use of intermediate synchronous condensers andvoltage regulators to compensate for a large part of the reactance ofthe synchronous condenser machines, particularly during steady-stateoperation, is given in a paper entitled Long-distance power transmissionas influenced by excitation systems, by C. Concordia, S. B. Crary, andF. J. Maginniss, published in Transactions of the American Institute ofElectrical Engineers, vol. 65, pp. 974-986. While the limit ofsteady-state operation in transmission systems as discussed in thispaper is of relatively great importance, it has become evident that aneven more important limit is the transient limit which must besufliciently high to allow the system to withstand faults and transientdisturbances.

In order to increase the transient limit of a system to the point ofrated power transfer, it is generally necessary to compensate the systemsufficiently either with series capacitors or intermediate synchronouscondensers to provide a relatively high steady-state stability limit. Inthe case of series compensation theseries capacitors must withstand thetransient swing current and the transients due to the occurrence andremoval of a fault, as well as transients produced by protectiveapparatus associated with In such an arrangement,

2 a line with high voltage protective equipment and the capacitors mustbe provided with a relatively high insulation to ground.

For transient conditions of the above-mentioned type the use ofintermediate synchronous condensers in a compensating arrangement hasmany advantages, and is generally desirable from a practical standpoint.lhe principal problem in the use of intermediate synchronous condensersis that of maintaining the reactance, comprising the synchronouscondenser transient reactance plus associated transformer leakage.

tion but must be available in case of departing; Thus if means may beprovided in an intermediate synchronous:

from steady-state conditions.

condenser branch to reduce the effective react-- ance of the condenserduring line disturbances:

the rating of the condenser is principally determined by transientconditions.

It is the principal object of this invention to, provide a new andimproved arrangement making use of intermediate synchronous condensers:in combination with certain impedance elements; to reduce the effectivesynchronous condenser re-- actance in the compensation of transmission.lines.

Broadly speaking, the invention provides impedances connected in serieswith intermediate synchronous condensers in a transmission line toreduce the effective synchronous condenser reactance during transientconditions. According to a preferred embodiment, series capacitors areconnected in the intermediate synchronous condenser branch of atransmission line. The series capacitors may be arranged to be switchedinto the circuit upon occurrence of a line disturbance by the use ofconventional protective devices such as relays. According to amodification of the above-mentioned embodiment, resistances areconnected in shunt relation with the series capacitors to providegreater compensation without danger of self-excitation than is providedby the use of capacitors alone.

The invention will be better understood by reference to the followingdescription taken in connection with the accompanying drawing and theappended claim, in which the features of the invention believed to benovel are more particularly set forth.

In the drawing Fig. l is a one-line schematic diagram of a transmissionsystem provided with intermediate synchronous condenser branches inwhich capacitors are connected in series with the synchronous condensermachines. Fig 2 is a modification of the arrangement of Fig. 1 with theaddition of resistances connected in shunt relation with the seriescapacitors.

Referring now to Fig. 1, there is shown an electrical transmissionsystem I interconnecting a generator 2 and a receiving system 3, throughsuitable step-up and step-down transformers 4- and 5 respectively.Transmission system: i isrepresented as comprising a plurality ofsections 6, 'I and 8, each having a pair of parallellines 9 andincluding suitable switchgear elements I'll. sections 6 and 8 are theend sections of transmission system I and accordingly are connected totransformers d and 5 respectively. Section I is the mid-section ofsystem I and is connected to sections 6 and Beta pair of switchingstations I I and, I1. respectively.

Stations. IrIaand i2 serve as. connection points for of intermediatesynchronous condenser branches t3 and i i.- which are generally similarin construction. Each branch comprises a stepdown. transformer it, asynchronous condenser 1.6,- a-seriescapacitor I'I, a protective gap I8and a switching element is. Synchronous condenser Ia is: connected-1 totransformer i5 through series capacitor I l. Transformers I5'of branchesIB-and it are connected in shunt relation withsystem I at points Handi2respectively'.

Ineach branch protective gap I3, which may be of a conventional type topro'ect capacitor I'I against voltage. surges and the like, is connectedin shunt relation with capacitor Il. Switching element I9, which islikewise connected-inshunt relation with capacitor- II, maybe thecontacts ota suitable associated protective device of conventionaltypesuch as a relay (notshown); which isarranged to function in response tovarious types of faultconditions.

It will: be: understood that the use of synchronous condenser branchessuch as branches I3 and Ill without the inclusion-of series capacitorsI1, gaps I3. and switch elements it; is well known. Such branches areemployed to improve. system stability and. provide required reactivesystem excitation for power factor correction and voltage regulation. Bythe addition of series capacitors the transient stability provided bythe use of intermediate synchronous condenser branches may befurtherimproved. It is pointed out in the above-mentioned paper thatforrelatively high'line loadings the. use of intermediate synchronouscondensers ismore eiiective if the total condenser capacity isdistributed along the line inbranches located at twoor three stations.as inithe arrangement shown in Fig. 1.

In: the normal; operation of system i and branches I3.-and= I l,switchelements I9.are arranged to be. closed thereby short-circuiting orby-passing series capacitors: Ii and-permitting synchronous condensersI6 to have maximum effectiveness in providing steadyestate stability andreactive excitationfor system I. However, inresponse to-certaintransient conditions such as a faultfiiidn one line of section 9 ofsystem I either orboth of switch elements ls'mayrbe caused tolopen byaction of the protective devices associated therewith, thereby insertingeither or both of capacitors I "I in series with synchronous condensersI6 in branches I3 and Ila. The insertion of capacitors I'I effectivelyreduces the reactances of synchronous condensers I5, thereby providing arelatively high degree of transient stability in system I. Gaps I8 areeffective in protecting capacitors I'I against transient voltage surgeswhich: may accompany the occurrence and removal of fault conditions aswell as transients it will he understood that switching elements I9 maybe arranged to be maintained open during normaloperation of system I. Itis desirable, however, to include switching elements I9 in branches isand I4 regardless of whether capacitors II are to be switched, sinceelements I9 may be employed to efiect deionization of gaps I8 shouldbreakdown thereof be caused by voltage surges thereacross.

In the arrangement of Fig. l I have shown capacitors H as connected inseries with synchronous condensers I6. Although such capacitors may belocated at other points in the system, it is desirablethat they bearranged and located insuchamanner that they may effectivelywithstand-faultconditions and'subsequent swing conditions. An advantageof connecting capacitors in series with anintermediate synchronouscondenserbranch-is that the capacitors may be located on the secondaryor low-voltage side of the intermediate synchronous condensertransformers, as exempli-fijed-by transformers E5 in Fig. l. Thus-thecost'of insulation of the capacitors may be reduced incomparison withthe cost of high voltage series capacitors connected in the main currentor power i'iow path. Furthermore, capacitors: connected in shuntbranches are in general lessapt to-be taken out of service by protectiveapparatus due to faults in a transmission linethan wouldg otherwise bethe case, since only capacitors atthestation near the fault aregenerally required to be switched. An additional advantagein-the case'offaults involving ground is that capacitors 'connected on the secondaryside-of the synchronouscondenser transformers are not required to-carryground components of current.

While inthe use of' series capacitors connected in intermediatesynchronous condenser branches it is generally desirable to obtainmaximum compensation,- the degree ofcompensation obtainable-is-limitedby conditions of self-excitation in the transmission system. Suchlimitations are set forth in U. S. Patent 2,149,082 of R. C. Buell, S.B. Crary, J W. Butler, and C. Concordia, dated Februa-ry28, 1939, andassigned to the same assignee as the present invention. Referring now toFig. 2 there is shown a modification of the arrangement of Fig. 1' whichpermits a higher degree of compensation without the danger ofself-excitation than is obtainable by the use of capacitors-alone as inthe arrangement of Fig. 1.

element Iris-provided in shunt relation with series capacitor I7, gap I8and switching element I9 in branches I3 and I4. By the use of a shuntresistor of suitable value in combination with the series capacitor inthe condenser branch, the tendency for self-excitation to occur may bereduced to a minimum as set forth in the abovementioned patent. Thus ahigher degree of compensation may be secured than would otherwise be thecase.

As described in connection with the arrangement of Fig. 1, capacitors H,in the arrangement of Fig. 2, may be inserted in series with synchronouscondensers I6 either permanently or in response to disturbances insystem I, as may be desired, by the use of switch element I9. If it isdesired to insert capacitors I! only in response to fault condtions insystem I, switch 22 may be permanently closed or may be omitted frombranches I3 and I4, thus permanently connectin resistor 22 in shuntrelation with capacitor II. Switching element I9 is caused to open inresponse to the fault conditions, thereby inserting the shuntcombination of resistor 2|, capacitor I7 and gap la in series withsynchronous condenser IS in either or both of branches I3 or I4. Byinserting these elements only in response to a line disturbance the fullcorrective value of the intermediate synchronous condensers may beutilized under normal conditions. Losses occurring in shunting resistors2I during periods of disturbance are of relatively short duration andtherefore do not constitute a serious economic handicap.

Should it be desirable to have capacitors I9 permanently inserted inseries with synchronous condensers I6, switching elements I9 may bemaintained open as in the operation described in connection with Fig. 1.Under this condition it is generally desirable in order to avoidcontinuous losses in resistors 2|, to maintain switch elements 22 openunder normal conditions and to effect closing of elements 22 only inresponse to disturbances in system I.

By the use of capacitor elements or combinations of capacitor andresistor elements in connection with intermediate synchronous condensersin accordance with my invention, it will be seen that substantial gainsmay be realized in the transient stability provided for long-distancepower transmission systems, with accompanyin increases in the powertransmitting capacities thereof.

While I have shown and described a preferred embodiment of my inventiontogether with certain modifications thereof, it will be understood thatmy invention may well take other forms and I, therefore, aim in theappended claim to cover all such changes and modifications as fallwithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

In an electric power system, a synchronous condenser for exciting apower transmission line, a capacitor connected between the line and thecondenser, a resistor connected in shunt with the capacitor, andswitching means connected to short circuit both the capacitor and theresistor to provide maximum reactive excitation for the line andoperable to insert the capacitor with the resistor in shunt therewith inseries relation with the synchronous condenser to increase the transientstability limit of the line.

SELDEN B. CRARY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,738,726 Philip Dec. 10, 19291,914,428 Hildebrand June 20, 1933 1,973,538 Mofiett Sept. 11, 19342,162,488 Marbury June 13, 1939 2,363,898 Partington Nov. 28, 1944

